1. Field of the Invention
The present invention relates in general to packaging and mounting semiconductor devices and in particular to packaging and mounting semiconductor photo transmitter/receiver (transceiver) devices in linear arrays. Still more particularly, the present invention relates to packaging semiconductor devices in a linear array of photosensors having a total length much greater than the size of a substrate on which the devices are formed.
2. Description of the Prior Art
The prevalence of imaging systems in contemporary consumer electronics relies in large measure on the fabrication of photosensors composed of charge coupled devices and CMOS devices. The charge coupled devices serve as photosensitive elements, with photons (light) impacting the surface of the charge coupled device creating charge at the terminals of the device. While a single sensing element detects light only at a single point, an array of sensing elements can detect variations of light reflected off an object being imaged as a function of position in a line or plane, enabling electronic encoding of a digital image of the object. Arrays of charge coupled devices find wide use in electronic products ranging from digital cameras to telecopier (facsimile or xe2x80x9cfaxxe2x80x9d) machines.
In some products, a linear array of photosensors detects a narrow line (often one or two pixels wide) of light reflected from the object being imaged. Moving the sensors relative to the object and/or the object relative to the sensors by the width of the array creates an adjacent linear image, a process which may be repeated with the adjacent linear images (also called optical lines) combined to form a two-dimensional image. In the case of flat-bed scanners, the sensor array typically moves relative to the scanned page to capture the image, while fax machines generally move the paper across the sensors. Use of a linear array to capture two-dimensional images reduces system cost since fewer sensor units are required.
Linear arrays of photosensors are typically manufactured from strips of integrated circuits containing charge coupled devices and mounted on printed circuit boards. Currently, the integrated circuits are mounted in an unencapsulated silicon form, then connected to a power supply and other support electronics mounted on the printed circuit board through the use of gold bond wires. The bond wires are then protected utilizing liquid encapsulation or silicon gel. This technique, often referred to as xe2x80x9cChip On Boardxe2x80x9d technology, provides ease of manufacture in production of sensor arrays having charge coupled devices as well as other integrated circuit designs.
However, Chip On Board technology causes removal and replacement of a defective integrated circuit segment from a linear array to be prohibitively difficult. Simple removal of an integrated circuit segment from a linear array for replacement is simply not possible in an economical manner. An array containing a defective segment must therefore simply be discarded. As the length of linear arrays increases, and the array includes more integrated circuit segments, the likelihood of at least one segment being or becoming defective increases substantially. Therefore, maintaining an acceptable yield in the linear array production constrains the length of arrays, a problematic limitation in that the number of parts and manufacturing steps in producing devices utilizing the linear arrays is increased. The limitation on array length also requires use of multiple linear arrays where designers would prefer a single linear array of greater length. Furthermore, multiple arrays must be aligned relative to each other during assembly.
It would be desirable, therefore, to produce linear arrays with greater length and more easily replaceable integrated circuit segments.
In linear arrays of charge coupled device photosensors, the sensor integrated circuits are contained in surface mountable packaging allowing individual segments to be soldered into place within the array. For solder-mountable packaging, unencapsulated sensor circuits are mounted onto a lead frame strip with the space between the circuits equaling the width of a singulation saw. After die mounting and wire bonding, a continuous strip of plastic or resin molding covers the wire bonds on one side and the edge of the silicon on the other, protecting the lead frame strip and other parts, leaving the active sensor area exposed. The lead frame is then trimmed and formed in a conventional manner, and the packaged sensor circuits are separated with a singulation saw cutting between the circuits. The resulting self-contained device may then be mounted within a linear array with solder rather than depending on Chip On Board technology. Leads are preferably soldered to the board on only one side, with the other side floating freely over the appropriate contacts for ease of mechanical adjustment. Individual sensor segments within the array may be readily removed and replaced in the event of a defect.